1. Field of the Invention
The present invention relates to an optical disk and an optical disk reproducing apparatus.
2. Description of the Related Art
FIG. 1 shows a part of an optical disk 2. The optical disk 2 has a substrate 3 having an information recording surface 5, and a protection layer 4 covering the information recording surface 5. A plurality of information pits 1, which have lengths modulated by recording information, are recorded on the information recording surface 5. During an information reproducing operation, a laser beam is projected onto one of the information pits 1 so that a laser beam spot LS having a predetermined diameter is formed on the information recording surface 5. A diffracted and reflected light is detected by a photodetector formed with, for example, a photodiode. The photodetector converts the received light into an electric signal. Information, such as audio information and video information, is extracted from the electric signal by means of a signal conversion process, which is the reverse of a signal conversion process for recording information. Examples of optical disks as described above are compact disks (CD) and laser video disks (LVD).
As shown in FIG. 2, the information recording density of optical disks greatly depends on the value of a track pitch P3 and the diameter of the laser beam spot LS projected onto the information pit 1. The track pitch P is the distance between the center lines of adjacent pit strings, that is, tracks. Hence, it is important to reduce the track pitch P in order to increase the information recording density.
In the cases as shown in FIGS. 1 and 2, there is no problem. However, as shown in FIG. 3, if the track pitch is set to a value P4 smaller than the value P3 in FIG. 2, to a certain extent (for example, P4=P3/2), information pit 1B or 1C on the adjacent track is concurrently included within the laser beam spot LS which is scanning an information pit 1A. In this case, a large quantity of crosstalk occurs, and hence the optical disk shown in FIG. 3 is not suitable for practical use.
In order to increase the information recording density, it may be effective to diminish the size of the laser beam spot LS. As shown in FIG. 4, the minimum beam diameter w obtained by focusing, by means of an objective lens OL, a laser beam having a wavelength .lambda. at a position corresponding to a focusing distance f of the objective lens OL is expressed as follows: EQU w=1.22.times.(.lambda./NA) (1)
where NA denotes a numerical aperture of the objective lens OL. The numerical aperture NA is defined as follows: EQU NA=n.times.sin .theta. (2)
where n denotes the refractive index of the objective lens OL, and .theta. is the angle of emergence from the objective lens OL. Hence, in order to reduce the diameter w of the laser beam spot LS, it is necessary to reduce the wavelength .lambda. or increase the numerical aperture NA. The wavelength .lambda. of a semiconductor laser used for optical disks is approximately equal to 0.780 .mu.m (the .mu.m is equal to 1.times.10.sup.-6 m). The numerical aperture NA for compact disks is approximately equal to 0.45. Hence, the diameter w of the laser beam spot LS is approximated as follows: EQU wmin=1.22.times.(0.780/0.45)=2.1 (.mu.m).
As a result, the minimum track pitch which does not result in crosstalk when the laser beam spot LS is projected onto a pit string is approximately equal to 1.6 .mu.m, and optical disks employ this value of the minimum track pitch.